RedTeam Pentesting discovered that rogue updates trigger a remote code
execution vulnerability in WebClientPrint Processor (WCPP). These
updates may be distributed through specially crafted websites and are
processed without any user interaction as soon as the website is
accessed. However, the browser must run with administrative privileges.

Neodynamic's WebClientPrint Processor is a client-side application,
which allows server-side applications to print documents on a client's
printer without user interaction, bypassing the browser's print
functionality. The server-side application may be written in ASP.NET or
PHP while on the client-side multiple platforms and browsers are
supported.

Upon installation under Microsoft Windows, WCPP registers itself as a
handler for the "webclientprint" URL scheme. Thus, any URL starting with
"webclientprint:" is handled by WCPP. For example, entering

webclientprint:-about

in the URL bar of a browser opens the about box of WCPP.

During RedTeam Pentesting's investigation it turned out that WCPP
supports an undocumented update mechanism. Through the following URL the
update mechanism is triggered:

Next, the XML file is parsed and the updated version of WCPP is fetched
from the URL

http://legitimate.example.com/somedir/wcpp.exe

with the returned byte stream being written to

C:\Program Files (x86)\Neodynamic\WCPP for Windows\v2.0\wcpp.exe

on a Windows 7 x86_64 machine. For the write process to succeed,
elevated or administrative privileges are required. Thus, the browser
which invokes WCPP must run with elevated or administrative privileges.

Proof of Concept
================

An attacker may prepare a malicious website containing the following
HTML code:

If the malicious website is visited by a WCPP user, the WCPP handler
(wcpp.exe) of the user's machine is replaced by code arbitrarily chosen
by the attacker. A visual indication of the update progress is displayed
and the success is indicated through a message box. However, successful
exploitation requires no user interaction. Any subsequent invocation of
an arbitrary webclientprint URL will result in the execution of the
attacker's code. Thus, the attacker may deliver a second inline frame
containing a webclientprint URL in order to force immediate execution of
the attacker's code.

Workaround
==========

Affected users should disable the WCPP handler and upgrade to a fixed
version as soon as possible.

Fix
===

Install a WCPP version greater or equal to 2.0.15.910[0].

Security Risk
=============

If a WCPP user visits an attacker-controlled website, the attacker may
execute arbitrary code on the machine of the victim user. However,
successful exploitation is only possible if the browser is running with
elevated or administrative privileges. On modern Microsoft Windows
systems, this is a rather strong prerequisite. Furthermore, the update
process is indicated on the user's screen, potentially causing
suspicion.

If successful, the attacker gains administrative privileges as well. A
skilful attacker may restore the original WCPP immediately while
migrating the malicious code to another place. This way, WCPP
functionality would not be disrupted and the attacked users may be
tricked to believe that a legitimate update has just occurred.

Because of the rarely fulfilled prerequisite of a browser running with
elevated or administrative privileges, this vulnerability is estimated
to pose a low risk.

RedTeam Pentesting offers individual penetration tests performed by a
team of specialised IT-security experts. Hereby, security weaknesses in
company networks or products are uncovered and can be fixed immediately.

As there are only few experts in this field, RedTeam Pentesting wants to
share its knowledge and enhance the public knowledge with research in
security-related areas. The results are made available as public
security advisories.

More information about RedTeam Pentesting can be found at:
https://www.redteam-pentesting.de/

Working at RedTeam Pentesting
=============================

RedTeam Pentesting is looking for penetration testers to join our team
in Aachen, Germany. If you are interested please visit:
https://www.redteam-pentesting.de/jobs/